1. Field of the Invention
This invention relates generally to high intensity illumination systems and more particularly relates to optical transmission systems using fiber optic light guides to carry light from high intensity, high temperature light sources. More specifically, this invention relates to apparatus and methods for coupling light from a high intensity light source into low temperature optical fibers.
2. Description of the Related Art
In the field of fiber optic transmission systems, it is known to use high intensity, high thermal output light sources such as, for example, mercury arc, metal halide arc, or xenon arc lamps, which have typical operating power in the range of 35 to 1000 watts. See U.S. Pat. No. 4,757,431, issued Jul. 12, 1988 and assigned to the same assignee herein. These light sources are used with a fiber optic light guide that may consist of a single fiber or a bundle of many small fibers. Standard fiber bundles typically consist of low melting temperature glass in contrast to fused silica or quartz for which the melting temperature is approximately 1000.degree. C. higher. Such systems have particular use in medical and industrial applications and are used in conjunction with instruments such as endoscopes, borescopes and the like.
Coupling light from a high intensity light source into a light guide requires the condensing and focusing of light, and its concentration results in a high power density at the focal point. The temperature rise at the focal point depends on the extent to which the light is absorbed. Larger spot sizes are associated with lower temperature rise; a small degree of absorption will result in a large increase in temperature. To reduce the temperature rise, the power density must be reduced as associated with larger spot sizes. To prevent a fiber bundle from melting, IR filters are typically placed between the light source and the bundle. As the focal point decreases in size, higher melting temperature materials such as quartz become necessary. As noted in U.S. Pat. No. 4,757,431, efficient methods exist for focusing the light down to a diameter of 1 mm or smaller and result in a much higher power density at the optical fiber target than is found with illumination systems delivering light through a fiber bundle. Such high power densities require light guides consisting of higher melting temperature materials to prevent melting of the optical fiber at the point of coupling of the light into the fiber. This applies to either single fiber light guides or small diameter (2 mm or smaller) fiber bundles.
Optical fibers made of quartz are expensive, and it is necessary that such optical fibers be used over a sufficiently long time period to justify their cost. In a surgical environment, this means that such optical fibers be sterilized after each use. Since sterilization techniques typically involve the use of high temperature autoclaves or chemical disinfectants, the optical fiber optic light guides must be made to withstand thermal damage and damage from the use of such chemicals. Additionally, quartz fibers are relatively brittle and difficult to bend without breaking, requiring a high degree of care during handling.
Although standard glass (e.g. borosilicate) fiber bundles are made of relatively inexpensive materials, their performance in transmission over long fiber lengths is limited by the transmissivity of the materials and packing losses. In addition, the low melting temperature of the glass places limitations on the smallest size bundle that can be coupled to a high intensity light source.
Coupling a fiber optic device, such as a microendoscope having an illumination aperture of 2 mm or less, to a typical light-delivering fiber bundle 3 to 5 mm in diameter is inefficient and results in poor light transmission to the optical device. The inefficiency arises from the mismatch in area. Reducing the size of the fiber bundle to match that of the device causes substantial coupling losses from the source, while narrowing the focus to a small diameter bundle results in melting of the bundle.
In general, the size of the light guide coupled to a fiber optic device should be matched to the diameter of the device. Hence for small diameter fiber optic devices (e.g. less than 2 mm) a single high temperature fiber or high temperature fiber bundle is necessary. Single fiber light guides having a diameter of 1 mm or less coupled to a source of light are more efficient than a bundle of similar size since bundles have inherent packing losses. Because single quartz or glass fibers over 1 mm diameter are generally too stiff for practical use, fiber bundles are typically used for applications requiring diameters greater than 1 mm.
Whereas single quartz fibers and glass fiber bundles are useful and effective in transmitting light, they are not the least expensive way of transmitting light. Plastic optical fibers are both inexpensive and highly flexible, even at diameters greater than 1 mm. Accordingly, it would be desirable to use these low cost plastic fibers in conjunction with high intensity light sources. Like glass bundles, however, plastic has a much lower melting temperature than quartz. Therefore, use of a single plastic fiber to deliver sufficient illumination requires an intermediate light delivery system between the plastic fiber and the light source.
One example of an application in which low cost plastic fibers or small diameter, lower cost glass fiber bundles would be useful is the medical field. Use of low cost fibers would enable light guides for lighted instruments in medicine to be sold as a single use, sterile product eliminating the need for sterilization after each use. The use of small bundles coupled to a single, high intensity quartz fiber would enable smaller devices to be manufactured. However, neither plastic fibers nor small diameter glass fiber bundles can withstand the high temperature generated at the focal point of a light source which is condensed and focused into a small spot commensurate in size with the diameter of such light guides.
U.S. Pat. No. 4,986,622 issued Jan. 22, 1991, discloses one prior art attempt at solving the problem of avoiding thermal damage to low temperature plastic fibers. The '622 patent discloses a light transmission apparatus coupling a heat resistant glass fiber optic bundle at the output of a high intensity light source. The glass optical fiber bundle is then mechanically close-coupled to a plastic fiber optic bundle in a standard connector. The '622 patent requires a mechanical matching of the glass fiber bundle to the plastic fiber bundle in order to avoid the generation of a significant amount of heat at the coupling point, which would damage the plastic fiber bundle.
The '622 patent requires that the diameter of the glass bundle be less than or equal to the diameter of the plastic bundle. This is to allow the cone of light emanating from the glass bundle to be transmitted into the plastic bundle without light loss. In practice, however, this is efficient only if there is also an optical specification with respect to the cone-angle of light (i.e., numerical aperture NA) for each bundle or optical fiber and the spacing between them. The '622 patent fails to recognize this requirement. Moreover, if the diameter of the glass bundle were significantly smaller than that of the plastic bundle, thermal damage to the plastic fiber would result at high power densities if a sufficient amount of light were coupled from the light source.
In the context of the '622 patent, it appears that typical 3 or 5 mm diameter bundles are used, since the connection between glass and plastic fiber bundles is that typically found in medical lighting equipment. Such connectors make use of a proximity coupling between fiber bundles with minimal spacing at the junction and rely upon a matching of the relative diameters of the bundles. For higher power densities, such connectors would cause damage to the low melting temperature fiber bundle.
Additionally, the '622 solution is insufficient to maximize light output from a low temperature fiber coupled to a high temperature single fiber delivering light from a high intensity source.
There remains a need in the art for improvement in methods and apparatus for coupling high intensity light into low melting temperature optical fibers.